Navigate to the the Language Overrides Manager. To add a new Override, click on the New icon in the toolbar. To edit an existing Override, click on the Overrides Constant or check the Overrides checkbox and press the Edit icon in the toolbar.
This is where you add a new Language Override String or edit an existing one.
A language string is composed of two parts: a specific language constant and its value. For example, a typical string you might find in a language file might look like:
COM_CONTENT_READ_MORE="Read more: "
Here 'COM_CONTENT_READ_MORE' is the constant and 'Read more: ' is the value. You have to use the specific language constant in order to create an override of the value. Here, you can search for the constant or the value you want to change with the search field below and it will display a list of constants or values that match your query. You can then click on the constant to automatically inserted it into the form.
At the top right you will see the toolbar:
The functions are: